Crystal hydrophone



y 1961 LE ROY c. PASLAY ETAL 2,983,901

CRYSTAL HYDROPHONE Filed April 16, 1945 2 Sheets-Sheet 1 gnaw (MAI L.CIPasZay LYBZBrown NNESZ es a www May 9, 1961 LE ROY c. PASLAY ETAL2,983,901

CRYSTAL HYDROPHONE Filed April 16, 1945 2 Sheets-Sheet 2 1/ I): III/4 #5C. B Brown JV. N E8188 CRYSTAL HYDROPHONE Le Roy C. Paslay,4012University Blvd., Dallas, Tex.;

'Charles B. Brown, 114 Forrester St. SW., Washington,

This invention relates to a hydrophone and more particularly to ahydrophone adapted to receive underwater signals having signalfrequencies within a predetermined frequency band and transmitelectrical-impulses corresponding thereto in which the strength of thereceived signals is multiplied by the hydrophone during the reception ofthe signals thereby to effecta, high degree of sensitivity of thehydrophone.

In devices of this character heretofore devised, it has i been the usualpractice to employ a diaphragm in communication with the surroundingwater of substantially the same area as the face of the crystalmicrophone in which'the diaphragm either is in physical contact with theface of the crystal element of the hydrophone or separated therefrom bya pressure transmitting fluid. These hydrophones are adapted to applysubstantially the same pressure to the face of the crystal microphone asthe pressure of the signals or impulses received by the diaphragm. Suchdevices have proved not altogether satisfactory under certain conditionsof service when employed with underwater ordnance devices, for example,by reason of the fact that the voltage generated by the crystalmicrophone in response to weak signals received is insuflicient tocontrol the electroresponsive mechanism connected thereto. 7 p

In the device of the present invention this difliculty has been overcomeby employing a diaphragm of considerably greater area than the pressuresurface ofthe crystal microphone and arranging a movable bafiie sectionor diaphragm of substantially rigid construction intermediate the firstnamed diaphragm and crystal microphone, and filling the space betweenthe two diaphragms and about the crystal with a pressure transmittingfluid, a vent or bypass being provided within the baflie plate to permita flow of fluid therethrough for a purpose which will become moreclearly apparent as the description proceeds. The bafile plate isprovided preferably with a pressure surface of substantially the samearea as the area of the flexible diaphragm and arranged adjacentthereto, and a second surface of reduced area correspondingsubstantially to the-area of the pressure face of the crystalmicrophone.

The strength of the pressure impulses received by the hydrophone is thusmultiplied and the increased pressure is applied to' a relatively smallcrystal microphone, thereby providing an arrangement in which theelectrical signal output of the microphone is greatly increased.

The device of the present invention is also adapted to respond tosignals within a predetermined frequency band such, for example, as arange of frequencies from 150 cycles to' 15,000 cycles per second, thelower limit of the frequency band being controlled by the size of theby-pass or vent withinthe baflie plate and the density of thepressuretransmitting fluid. By employing a vent of sufficiently smallerdiameter, the lower range of frequencies to which the device isresponsive may be extended to a predetermined frequency cut-ofisuch, forexample, as "5 cycles per second. Furthermore, by employing the PatentedMay 9, 1961 arrangement disclosed herein, the hydrophone may be designedin a compact unit which is economical to manufacture and which possessesa high degree of sensitivity.

. One of the objects is the provision of new and improved means withinthe hydrophone for increasing the strength of the impulses received andapplying the increased signals to a crystal microphone element.

Another of the objects of the present invention is to provide asensitive hydrophone in which the crystal element thereof is relativelysmall.

Another of the objects is the provision of a hydrophone having new andimproved means for controlling the frequency band of the signals towhich the hydrophone is responsive.

A still further object is the provision of a hydrophone having new andimproved means for preventing the response of the crystal microphoneelement thereof to frequencies of less than a predetermined frequency.

Still another object is to provide a hydrophone having new and improvedmeans for causing the hydrophone to be uniformly sensitive to "incomingsignals throughout a predetermined range of frequencies.

A further object is the provision of a hydrophone for use for variouspurposes such, for example, as for controlling the operation of marinemines, torpedoes, seismic detecting devices and the like, which will berugged in construction, economical to manufacture and which possessesall the qualities of reliability and a high degree of sensitivity inservice.

Still other objects, advantages and improvements will become apparentfrom the following description taken in connection with the accompanyingdrawings, of which:

Fig. 1 is a front view of the device of the present invention inaccordance with a preferred embodiment thereof and showing a fragmentaryportion of the casing to which the device is secured;

Fig. 2 is a rear view of the device of Fig. 1 with the cover partiallybroken away;

Fig. 3 is an enlarged end sectional view in elevation of the-device;

Fig. 4 is a 'view partially broken away of the device with the baffleplate removed; and Fig. 5 is a fragmentary view in section showing thefiller plug and the pair of signal conductors extending from the casingof the device.

Referring now to the drawings for a more complete understanding of theinvention, on which like numerals of reference are employed to designatelike parts throughout the several views and more particularly to Fig. 1thereof, there is shown thereon a casing 10 such, for example, as thecasing of the mine to which is secured as by a plurality of bolts 11, ahydrophone indicated generally by the numeral 12. The casing 10 isprovided with a circular aperture 13 within which the hydrophone isarranged. The hydrophone includes a circular support 14 having a raisedannular portion 15 adapted to be brought into registered engagement withan annular recessed portion 16 within the casing, within which isdisposed a gasket 17 of material suitable for the purpose such, forexample, as rubber, thereby to effect a watertight seal between thesupport 14 and the casing as the bolts 11 are tightened. A plurality ofwashers 18 are preferably employed between the support and the heads ofthe bolts 11 to maintain the hydrophone secured to the casing and inwater-tight relation therewith.

The support 14 is provided with a circular recessed portion 19 having alip 21 arranged thereabout and adapted to be engaged by a flexiblediaphragm 22 composed of rubber or any of the synthetic varietiesthereof thereby to effect a seal between the diaphragm and the support14 as the diaphragm is clamped thereto by the clamping ring 23 andscrews 24. Preferably, though not necessarily, there is provided beneaththe heads of the screws 24 a plurality of washers 25. The support alsoincludes an annular recessed portion 26 thereby to provide a baflieplate or diaphragm 27 of truncated conical shape, in the form of afrustum of a cone with a base of substantial breadth and a relativelysmaller altitude, concentrically arranged within the recessed portion 26of the support and integrally connected to the support by a thin annularportion 28 of the support whereby the baffle plate 27 is adapted to beactuated in an axial direction in response to impulses of pressurereceived by the rubber diaphragm 22 by reason of the thin annularportion 28 which supports the baflie plate. There is also arrangedwithin the baffle plate 27 an aperture 29 within which is arrangedv abushing or sleeve 31, The sleeve 31 is secured to the bafile plate inany convenient manner as by pressing the parts together and provides aby-pass or vent between the inner and outer faces of the baflie platefor the flow of fluidtherebetween as pressure impulses are received bythe diaphragm 22, the rate of flow of the fluid being controlled by thesize of the openingwithin the sleeve 31. 1 1

The support 14 has secured thereto as by the bolts 32, a casing 33,generally circular in shape, and provided with a Well 34 within which isarranged a crystal microphone indicated generally by the numeral 35.

A suitable gasket 36 is disposed within complementary annular recessedportions of the support 14 and casing 33 thereby providing an oil-tightjoint therebetween when the bolts 32'are drawn tight, the tightness ofthe joint being effectively maintained by a plurality of washers 37arranged beneath the heads of the bolts 32' The cavity on the inwardsideof the diaphragm 27 is partly filled by an arcuately shaped member 38composed of material suitable for the purpose such, for example, asamaterial known in the trade as Corprene, to which a flexible coating ofoil resisting compound has been applied.

The crystal microphone may be composed of any material suitable for thepurpose such, for example, as ammonium dihydrogen phosphate whereby thefrequency response of the crystal is preferably independentof thetemperature of the crystal and the crystals possess the characteristicof being relatively insensitive to variations in humidity.

The crystal elements comprising the microphone comprise a plurality oflaminae of the crystal material'secured together in a unitary structureas by a coating of cement applied thereto and by a length of cord 39wrapped thereabout. The external electrical connections to the crystalare established by a pair of conductors 41 and 42 secured to the pads 43and 44 respectively whereby alternate crystals are connected to one ofthe conductors and the remaining crystals are connected to the other oneof the conductors. The crystal microphone is cemented to a pair ofplates 45 composed of insulating material suitable for the purpose such,for example, as a material known as linen Bakelite, the plates being ofsuflicient thickness to cause a predetermined pressure to be applied tothe crystal when the casing 33 is clamped to the sup? port 14 and noimpulses are received by the flexible diaphragm 22. e

The conductors 41 and 42 are connected in any suitable manner to a pairof rods 46, each of the rods passing through a bushing 47 arrangedwithin an aperture 48 within the casing 33 substantially as shown. Agasket or gland 49 is also arranged about each of the rods 46 andadapted to be compressed by the bushing 51 and nut 52 sufiiciently toform an oil-tight seal between the rod 46 and the casing. An externalelectrical connection to the rods 46 is established by-the-pair ofconductors 53 secured thereto. 5

There is also provided within the casing 33 an annular recessed portion54 adapted to be enclosed by the cover 55 secured to the casing as by ascrew 56.

The conductors 53 extend through an insulated bushing 57 arranged withinthe casing 33 and are preferably formed into a knot 58 within therecessed portion 54 of the casing, as is well known in the electricalart. There is also provided within the casing 33 a filler hole 59,through which the interior cavities of the hydrophone are filled with afluid such, for example, as a fluid known in the trade as Dow Corning,having a viscosity which varies .but slightly with changes intemperature. A sufficient quantity of this fluid is introduced into thecavities of the hydrophone to force the flexible diaphragm22 outwardly,substantially as shown in Fig. 3.

A convenient method of achieving'this result is to reduce the pressureon the outside of the flexible diaphragm 22 during the filling operationalthough, if desired, the fluid may be inserted into the aforesaidcavities under pressure. The filler hole 59 is sealed by a gasket 61arranged between a pair of discs 62 and adaptedto be compressed by 'theplug '63 sufficiently to effect an oiltight seal.

The cover 55 is provided with an aperture 64 through which a quantityof'material known as insulating compound is preferably introduced withinthe annular recessed portion 54 of the casing 33 sufficiently to form anadditional seal for the electrical terminals and-filler hole andadditionally to secure the conductors 53 to the casing.

By providing a 'suflicient quantity of fluid within the cavities of themicrophonesuflicient'to effect a considerable distension of the-flexiblediaphragm 22 from the baflle plate 27 before submersion of the devicewithin the water, the diaphragm is prevented from striking the baffleplate while impulses of pressure are received by the device-while ina-submerged condition.

By providing abaffle plate having a rigid truncated cone-shaped centralportion, supported by a'thin annular section in such a manner that thebase of the baffle plate is disposed adjacent the flexible diaphragm 22and the reduced area of the plate is in abutting relation with thecrystalrnicrophone assembly, an arrangement is provided such that amechanical advantage-is gained by exerting the combined pressure on thebase of the diaphragm -against-the crystal assembly and thus thepressure per unit area applied to the crystal is much greater-than thepressure'perunit' area applied by the incoming signalto the flexiblediaphragm 22. Furthermore, by providing a 'baflle plate or diaphragm 27comprising a solid central portion and an annular weakenedportion at theouter periphery of the diaphragm of relatively-small compliance orresistance with respect to the central portion of the diaphragm therestoring force applied to the central portion of thediaphragm by theaforesaid thin annular section is sufliciently small as to be negligiblein comparison with the force transmitted from the diaphragm to thecrystal assembly;

An arrangement is thus provided in which a chamber 65 intermediate thebase ofthe baflie plate and the flexible diaphragm 22 is incommunicationwith a chamber on the opposite side of the plate 27 by a duct formed bythesleeve 31 within the solid portion ofthe baflle plate. When thepressure applied to the outside of the flexible diaphragm 22 isincreased, the fluid withinthe chamber 65 is; additionally compressed,thereby causing a flow of fluid from the, chamber 65 through theduct31.into the chamber 66, and, conversely, when the pressure against theoutside of the flexible diaphragm .is reduced, the fluid flows from thechamber 66 into the chamber .65.. The combinationof the density offluid, the size of the aperture 31, andthe compliance of the member 38within the chamber 66 causes a resonant condition at a predeterminedfrequency, which may increase the degree of re,- sponse of thehydrophone to signals of that frequency and produce a sharp reductionofv response to signals having frequencies lower than the resonancefrequency. This condition is caused by the difference in theinstantaneous pressure of the'fluid within the chamber 65 on the base ofthe'baflie plate and the pressure of the fluid the a chamber- 66 againstthe inside portion-of the balfle plate and the active faces of thecrystal which are not normally or but slightly pressed otherwise.

The device is particularly adapted for use with underwater ordnanceemploying acoustic detecting devices in which it is desired toreceivesignals of relatively weak strength within a predeterminedfrequency range such, for example, as the range from 150 cycles to15,000 cyclesper second. The device, howevenis also well adapted for usewith signals having a different range of frequencies such, for example,as the range from to 18 cycles per second, it being merely necessary toemploy a bushing within the baflle plate having an aperture therein ofsmaller size to providethe desired cut-off characteristic of the deviceto signals of the lower frequency. Conversely, by employing a bushingwithin the baflle plate having a larger aperture therein, the device isadapted to cut olf signals having frequencies of less than a relativelyhigher frequency. Furthermore, by employing a pressure multiplyingbattle plate having a weakened annular section on the periphery thereof,the voltage generated by the crystal assembly is increased in proportionto the ratio of the multiplication factor of the aforesaid baflle plateand thus is made substantially equal to the sensitivity and energyoutput of a crystal assembly of much greater size and-havingsubstantially the same pressure surface area as the exposed area of theflexible diaphragm in communication with the surrounding water.

While the invention has been described with reference to a specificembodiment which gives satisfactory results, it will be understood bythose skilled in the art to which the invention appertains, afterunderstanding the invention, that various changes and modifications maybe made without departing from the spirit and scope of the invention,and it is our intention, therefore, in the appended claims to cover allsuch changes and modifications.

The invention herein described may be manufactured andused by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

What is claimed as new and desired to be secured by Letters Patent ofthe United States:

1. In a crystal hydrophone of the character disclosed, in combination, ametallic casing having a planar flexible annular portion, a rubberdiaphragm secured to said casing and in communication with thesurrounding water, a pressure multiplying device within the casing ininterposed relationship between the diaphragm and the interior of thecasing and adapted to be actuated variably in accordance with thestrength of impulses received by said diaphragm, said device comprisinga rigid member formed integrally with said flexible annular portioncentrally thereof and having a broad flat base coplanar therewith, saidmember being in the form of a frustum of a cone of small altitudeterminating in a surface parallelto and of less area than the base, anda crystal microphone within the interior of the casing in abuttingrelation with the terminal surface of said member and adapted to beactuated by the member as the pressure multiplying device is actuated inresponse to said impulses.

2. In a hydrophone of the character disclosed, in combination, ametallic casing including an exterior wall of predetermined thicknesshaving a planar circular portion on the exterior thereof, said wallinteriorly of said casing being formed with an annular recess coaxialwith said circular portion for providing a central inwardly-taperedmember connected to the remainder of the wall by a flexible annularportion, said member terminating in a surface of less area than the areaof the circular portion, said casing having a well therein opening infacing relation to said surface, means for transmitting pressureimpulses in the surrounding water to said member for actuation thereofin response to the impulses, and a crystal microphone supported withinsaid well in abutting relaaesaee secured to saidv casing and incommunication with the surrounding water, a movable bafile plate formedintegral ly with said annular portion and arranged intermediate saiddiaphragm and said well, said baffle plate having a surfaceadjcent saiddiaphragm and a second surface thereon opposite said well and of lessarea than the area of the first named surface, a crystal microphoneassembly supported within said well in abutting relation with saidsecond surface and adapted to receive pressure impulses from the baffleplate as pressure signals are received through the water by saiddiaphragm, a quantity of fluid within said casing for transmittingpressure impulses from said flexible diaphragm to said baflle plate, anda duct within said baffle plate for establishing a fluid connection.between said surfaces.

4. In a hydrophone of the character disclosed, the combination of ametallic casing, a flexible diaphragm in communication with thesurrounding Water and enclosing said casing, a movable baffle sectionwithin the casing, a flexible annular member formed integrally with bothsaid section and said casing for supporting said section therefrom, afluid connection between the baflle section andv said diaphragm wherebythe baflle section is adapted to be moved variably as impulses arereceived by the diaphragm, said baffle section being a frustum of a conehaving a broad flat base coplanar with said flexible annular memher. anda terminal surface of reduced area, and a laminated crystal microphoneinterposed between said casing and the baffle section, said microphonehaving a crosssectional area coextensive with and abutting said surface,said microphone being adapted to generate electrical impulses inresponse to variations in pressure impulses received by the microphonefrom said baflle section, said microphone comprising a plurality oflaminae arranged perpendicularly to said surface of reduced area andcomposed of crystal material possessing the characteristics of beingrelatively insensitive to variations in humidity.

5. In a hydrophone of the character disclosed, the combination of ametallic casing, a movable baffle section within the casing and adaptedto be moved variably as impulses are received by the section, a flexibleannular member formed integrallywith both said section and said casingfor supporting said section therefrom, said baffle section having afnisto-conical configuration including a broad flat base coplanar withsaid flexible annular member and a parallel surface of reduced areathereopposite, a laminated crystal microphone interposed between saidcasing and the baflle section in abutting relationship with said surfaceand adapted to generate electrical impulses in response to variations inpressure received by the microphone from said baffiesection, saidmicrophone compris ing a plurality of laminae composed of crystalmaterial disposed perpendicularly to said surface of reduced area meansincluding a pair of electrical conductors for establishing an externalelectrical connection to the hydro phone, and means for connectingalternate ones of said laminae to each of said conductors respectively.

6. In a hydrophone for detecting signals within a predeterminedfrequency range, the combination of a metallic casing provided with anintegral flexible annular portion and a chamber, a movable bafliesection substantially enclosing said chamber and formed integrally withsaid annular portion, a flexible diaphragm on said casing incommunication with the surrounding water and arranged adjacent saidbaflle section thereby to form a second charm ber, a supply of fluidwithin each of said chambers, a duct within said baflle section forcontrolling the flow of fluid between said chambers as the diaphragmvibrates in response to signal impulses received through the surroundingwater whereby the hydrophone is unresponsive to signals havingfrequencies of less than a predetermined frequency, and a crystal.microphone havingja' pressure facerlessthan the area of, saidflexiblediaphragm in abutting relation with said movable baffle section andadapted to generate electrical. signals corresponding respectively tosaidsignal impulses in response to pressure impulses received from: thebaflle section.

7. In a hydrophone for detecting. signals within apredet'erminedfrequency range, the combination of a metalhc casingprovided with an integral flexible annular portion andv a chamber,amovable, baflle section substantially enclosing said chamber and formedintegrally with sa d annular portion, a flexible diaphragm, on saidcasing m communication with the surrounding water and arranged adjacentsaid bathe section thereby to form a second chamber, a supply of fluidwithin each of said chambers, a duct within, said baffle section forcontrolling the flow of fluid between said chambers as the diaphragmvibrates in response to signal impulses received through the sur- 7rounding water whereby the hydrophone is unresponsive tosiguals havingfrequencies of less than a predetermined frequency, a crystal microphonehaving a pressure face less than the area of said flexible diaphragmarranged in abutting relation with said baflle section and adapted togenerate electrical signals corresponding respectively to said signalimpulses in response to pressure impulses received from the bafllesection, and means for displacing a volume of said fluid variably inaccordance with the pressure of the fluid within the second chamber.

8., In a hydrophone for detecting signals within a predeterminedfrequency range,the combination of a metallic casing provided with anintegral flexible annular portion and" a chamber, a movable bafiiesection enclosing said chamber and formed integrally with said annularportion, a flexible diaphragm on said casing in communication with. thesurrounding water and arranged adjacent said baflle section to form asecond chamber, a supply of fluid within each of said chambers, a ductfor controlling the flow of. fluid between said chambers as thediaphragm vibrates in response to impulses received through thesurrounding water thereby to control the low frequency cut-oiiof thesignals received, and a crystal microphone in said firstnamed chamberand'having a pressure face less than the area .of said flexiblediaphragm in abutting relation with said" bafllesection and adapted tobe actuated thereby.

9'. In a crystal hydrophone ofthe character disclosed, in combination, acasing having a chamber therein, a flexible diaphragm secured to saidcasing and in 001111 munication with the surrounding water, a planarflexible annular. portion on saidcasing having a pressure multiplyingvvdevice co-extensive therewith in interposed relationship between the,diaphragm and the chamber and adapted for variable actuation inaccordance with the strengthof impulses received by said diaphragm, saiddevice comprising a rigid central member formed integrally with said'flexible annular portion, saidmember having theform of a frustum of acone including-a broad flat base coplanar. with said annular portion anda surface parallel to. and of less area than saidbase, a microphoneinterposed in said chamber in abutting, relation with said surface ofsaid" pressure multiplying device and adapted to be actuated thereby asthe flexible diaphragm vibrates in response to said impulses.

' 10. The structureof claim 9' further 'charactcrized' in combinationtherewith, by an acoustic-impedance-match'-- ing coupling meansdisposedbetween'said' diaphragm: and said multiplying device forproviding sign-a1 communication. therebetween. V V

11'. The-structureoffthecombination of claim 10 further'including afrequency filter means'comprising a duct passing through said pressuremultiplying 'device for by passing signals of apredeterrninetlfrequency;

12'. The combination, in a hydrophone of the character disclosed, of acasing, a microphonein' said casing, a flexible diaphragm secured tosaid casing inenclosingrelationship therewith and in communication withthe surrounding water, a movable bathe section having a planar annularportion co-extensive with said casing whereby it is supported thereby,said baffle section being disposed" intermediate said diaphragrnand saidmicrophone and having a central portion in the form of a frustum of acone-with' a broad base anda' small" altitudeand thereby providing afirst plane surface thereof ad'- jacent said? diaphragmand a secondplane surface thereopposite ofless' area than the area ofsaid-first-named surface, said firstplanesurface beingcoplanar withsaidan- I nular portion, said microphone being. disposedbetween saidcasingand saidi-baflle section fon receptionof pressure impulses fromthe'baffle section as pressure" signals are received: through the waterbysaid' diaphragm and a fluid within said casingfor transmitting:pressure; impulses from said flexible diaphragm to said bafllesection.

13. The combinationnofclaim 12fuzther characterized by a low frequencybypass filter means comprising a fluid duct extending through saidbaflle section.

References Cited inthe file of this-patent UNITED STATES PATENTS-1,216,946' Clement Feb. 20, 1917 1,380,869 Fay June 7, 1921 1,591,068Williams July 6, 1926 2,044,807 Moyes June 23, 1936 2,088,324 John July27, 1939 2,233,992 Wyckofl Mar. 4, 1941 2,405,226 Mason Aug. 6, 19462,626,380 Stedman Jan. 20, 1953 FOREIGN PATENTS 168,839 GreatBritainMar. 26, 1918 852,150 France Jan. 24, 1940

